Conventional quasi-Doppler direction finding (DF) systems require that (1) the diameter of any supporting mast be small compared to the array diameter and (2) the spacing between adjacent antenna elements arranged in a circle be sufficiently small to ensure that the actual phase difference between signals from adjacent elements is less than 180.degree. in magnitude. A failure to meet either of these two constraints produces direct and sometimes large DF error. Relative to the first constraint above, any propagating wave incident upon a large mast supporting the array produces a scattered wave which interferes with the incident wave to distort the antenna pattern of each antenna element and to thereby produce errors in the measured direction of arrival of the incident wave. Relative to the second constraint above, the measured phase between any two signals is a cyclic function of delay between the two signals, with the cycle period being 360.degree. of phase. Consequently, phase measuring instrumentation is unable to discriminate between any given phase and a phase that differs by integer multiples of 360.degree. from the given phase. Properly designed phase measurement instrumentation provides accurate values of actual phase differences as long as the actual phase differences are less than 180.degree. in magnitude, but such phase measurement instrumentation produces very large phase errors (360.degree., 720.degree. or more of "phase ambiguity error") whenever the actual phase differences are bipolar and exceed 180.degree. in magnitude. As a result, whenever the separation between adjacent antenna elements produces actual phases that differ by more than 180.degree., severe DF error results. These phase ambiguities can be avoided in conventional quasi-Doppler systems by appropriately increasing the number of elements, thereby reducing the inter-element spacing and the inter-element phase. However, the increased number of elements contributes to increased system complexity, and can lead to problems from increased mutual coupling between the elements. Alternatively, the number of elements can remain fired and the array diameter decreased to reduce the inter-element spacing. However, decreasing the array diameter decreases direction finding accuracy and raises the lower operating frequency of the array.
The present invention provides accurate measurements of azimuth angle of arrival of a transmitted signal at an antenna array supported on a large central support mast and allows the actual phase difference between adjacent elements to exceed 180.degree. in magnitude. The ability to perform DF under the resulting condition of distorted antenna patterns and phase ambiguities results from observing that the actual phase and amplitude pattern distortions of the antenna elements are repeatable functions of frequency and can be exploited to eliminate phase ambiguities. The impact of pattern distortion on DF accuracy is minimized in the present invention by using Fourier processing to eliminate harmonic distortion in the antenna phase pattern.
The invention includes a circular array of N identical antenna elements arranged symmetrically about a central support mast. The signal from each element is processed to provide data points (i.e., samples) of the amplitude pattern and the unresolved phase (i.e., including possible phase ambiguity errors) pattern. The samples of the antenna patterns are processed to provide a coarse azimuth estimate of the radial line of symmetry for the array antenna pattern. The radial line of symmetry ideally points to the direction of arrival of the incident signal, and it provides a starting azimuth for resolving the 360.degree. ambiguities that are inherent in the phase measurements. After using the line of symmetry as an azimuth to approximately align the sample phase pattern with the expected shape of the distorted phase pattern, each phase measurement is resolved by adding or subtracting the necessary multiple of 360.degree. required to minimize the difference between the resolved phase measurement and the expected shape of the distorted phase pattern. The set of resolved phase measurements describing the distorted phase pattern is then filtered by use of Fourier analysis to eliminate harmonic distortion within the pattern. The measured value of azimuthal direction of arrival of the incident wave is determined as the azimuth at which the filtered phase pattern is maximum.
The present invention enables the N antenna elements to be installed on a support mast common to other antenna systems. The common through mast may support other antenna systems higher and/or lower on the through mast. As an example, a large through mast on a ship may support several antenna systems at different heights on the same mast.